1. Field of the Invention
From a general point of view, the present invention refers to a process of producing urea.
More particularly, the invention refers to a process of producing urea of the type comprising the steps of:
reacting substantially pure ammonia and carbon dioxide in a first reaction space, so as to obtain a first reaction mixture including urea and carbamate; PA1 subjecting the first reaction mixture to a thermal decomposition treatment of the carbamate so as to obtain a flow of ammonia and carbon dioxide and an independent flow of urea, undecomposed carbamate and free ammonia in aqueous solution; PA1 feeding the ammonia and carbon dioxide flow to a second reaction space; PA1 feeding the flow of urea and undecomposed carbamate in a urea recovery section so as to obtain independent flows of urea, ammonia and carbamate; PA1 recycling the flow of carbamate so obtained in the second reaction space; PA1 reacting ammonia and carbon dioxide obtained from the thermal decomposition treatment and the .recycled carbamate in the second reaction space, so as to obtain a second reaction mixture.
The invention also refers to a plant for carrying out the above mentioned process.
2. Description of the Related Art
As is known, in urea production, the need is more and more felt of having on the one hand plants with higher capacity and operational flexibility and, on the other hand, lower investment and operating costs, especially in energetic terms.
To this aim, various urea production processes have been proposed and reduced to practice in the art which are substantially based on conversion reactions carried out with different yields in reactors disposed in parallel one to the other, as described, for example, in European Patent Application EP-A-0 479 103.
More particularly, such processes propose to carry out a first reaction in a primary high-yield reactor fed by substantially pure carbon dioxide and ammonia and optionally by highly pure recovered ammonia, as well as a second reaction in an auxiliary lower yield reactor, in parallel to the preceding one, fed by a solution (substantially comprising unreacted carbamate) coming from a urea recovery section.
In a second known process, described for example in European Patent Application EP-A-0 544 056 by the same Applicant, the auxiliary lower yield reactor is fed not only by the solution coming from a urea recovery section, but also by ammonia and carbon dioxide obtained from a thermal decomposition treatment of the reaction mixture leaving the high-yield reactor.
Even if these processes and in particular the last one substantially attain to the aforementioned aim, they may be affected in operation by a certain inelasticity.
The operating conditions of the auxiliary reactor, in particular temperature and ammonia/carbon dioxide molar ratio, are in fact exclusively controlled by regulating the temperature of the thermal decomposition treatment of the reaction mixture leaving the primary reactor, with the result that to maintain the desired temperature in the auxiliary reactor, the ammonia/carbon dioxide molar ratio can shift from the optimal value and vice-versa.
The plant which carries out the above mentioned process is affected by a number of different structural constraints that make its manufacture more complex and expensive.
In order to ensure a correct feeding to the auxiliary reactor of the vapors obtained from the thermal decomposition of the reaction mixture leaving the high-yield reactor, in fact, the plant must be provided with specific structures which permit to position the auxiliary reactor at a higher level with respect to the equipments in which the thermal decomposition treatment takes place.